Fundamental difficulties arise in the attempt to measure the optical transfer function at high spatial frequencies. These difficulties are due to the basic nonlinearity of optical imaging systems at high spatial frequencies. The physical significance of this nonlinearity in image evaluation is illustrated by considering two commonly used methods of measuring the optical transfer function, i.e., the imaging of sine waves and edges. It is shown that at spatial frequencies considered to be within the state of the art, both of these methods lead to apparent, or measured, transfer functions which are significantly different from those which would be obtained for a linear system. For the practical case where the object mutual intensity is spatially stationary, it is found that the importance of this nonlinearity is determined by the ratio of the coherence interval of the object illumination to the size of the imaging system’s diffraction pattern. These nonlinearities can therefore become important at lower spatial frequencies. The implications of these results for image evaluation are discussed.
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